Due to security demands and technical limitations, electric values of high voltage cannot be directly measured in a power system. Both protection and measurement are carried out at low voltage side, that is, both protection and measurement are carried out after potential transformers (PTs) and current transformers (CTs), which are used to convert the high-voltage and large-current signals to and low-current signals in proportionally. As shown in FIG. 1, PT 1 is a voltage converter manufactured in accordance with a proportion Ku based on the principle of electromagnetic induction. The low voltage signal, which is reduced K times via PT1, can be sampled and thus the high voltage value can be measured indirectly.
Similarly, the large current of high voltage can be reduced Ki times to a low current signal in low voltage side via CT 2 and the large current of high voltage can be measured indirectly.
A meter or protector 3 is a low-voltage energy meter or protector unit, which samples the secondary signals of a PT and a CT and measures the electric energy within a certain time, or analyses the data of voltage and current, and then determines whether to give a protection signal or not.
The traditional high voltage power energy metering (HVEM) system and distribution network protector mainly have the following problems:
1) the insulation between high voltage side and low voltage side leads to the large size of a PT or a CT and a high consumption on copper, iron and insulating materials;
2) traditional high voltage PT and high-voltage CT adopted the electromagnetic measurement technology have hidden ferromagnetic resonance problem and therefore undermines the safety of power system;
3) as it is expensive to test CT at a high voltage situation, CT are generally tested at a low voltage situation, therefore, the CT's accuracy cannot be effectively controlled under high voltage situation;
4) The accuracy grade of traditional HVEM system cannot be determined clearly under existing error control methods, therefore, the power loss caused by measure errors cannot be effectively controlled, which will lead to a severe economic loss to the power generation and supply department as well as the user.
Although the novel electronic transformers have been widely applied in digital substations, which are mainly applied at a voltage level of 110 kV and above, due to the cost factor, now a large number of traditional transformers are used for measurement and protecting units in distribution network systems, and digital substations cannot be realized truly.
The measure and protection for existing distribution network are integrated in a complete switch cabinet and both need a separate metering cabinet, which leads to a great number of devices existing in the substation of a distribution network, a large energy consumption of devices and a large area occupied.